The present invention can be applied to a broad range of semiconductor devices, although it is especially effective in light-receiving elements such as photodiodes and solar cells. The background of the invention is described below with reference to solar cells as a specific example of the prior art.
A conventional solar cell structure with a p-type base has a negative electrode that is typically on the front-side or sun side of the cell and a positive electrode on the backside. It is well-known that radiation of an appropriate wavelength falling on a p-n junction of a semiconductor body serves as a source of external energy to generate hole-electron pairs in that body. Because of the potential difference which exists at a p-n junction, holes and electrons move across the junction in opposite directions and thereby give rise to flow of an electric current that is capable of delivering power to an external circuit. Most solar cells are in the form of a silicon wafer that has been metallized, i.e., provided with metal contacts that are electrically conductive.
Most terrestrial electric power-generating solar cells currently are silicon solar cells. Process flow in mass production is generally aimed at achieving maximum simplification and minimizing manufacturing costs. Electrodes in particular are made by using a method such as screen printing to form a metal paste. An example of this method of production is described below in conjunction with FIG. 1.
FIG. 1A shows a p-type silicon substrate, 10.
In FIG. 1B, an n-type diffusion layer, 20, of the reverse conductivity type is formed by the thermal diffusion of phosphorus (P) or the like. Phosphorus oxychloride (POCl3) is commonly used as the phosphorus diffusion source. In the absence of any particular modification, the diffusion layer, 20, is formed over the entire surface of the silicon substrate, 10. This diffusion layer typically has a sheet resistivity on the order of several tens of ohms per square (Ω/□), and a thickness of about 0.3 to 0.5 μm.
After protecting one surface of this diffusion layer with a resist or the like, as shown in FIG. 1C, the diffusion layer, 20, is removed from most surfaces by etching so that it remains only on one main surface. The resist is then removed using an organic solvent or the like.
Next, a silicon nitride film, 30, is formed as an anti-reflection coating on the n-type diffusion layer, 20, to a thickness of typically about 700 to 900 Å in the manner shown in FIG. 1D by a process such as plasma chemical vapor deposition (CVD).
As shown in FIG. 1E, a silver paste, 500, for the front electrode is screen printed then dried over the silicon nitride film, 30. In addition, a backside silver or silver/aluminum paste, 70, and an aluminum paste, 60, are then screen printed and successively dried on the backside of the substrate. Firing is then carried out in an infrared furnace at a temperature range of approximately 700° C. to 975° C. for a period of from several minutes to several tens of minutes.
Consequently, as shown in FIG. 1F, aluminum diffuses from the aluminum paste into the silicon substrate, 10, as a dopant during firing, forming a p+ layer, 40, containing a high concentration of aluminum dopant. This layer is generally called the back surface field (BSF) layer, and helps to improve the energy conversion efficiency of the solar cell.
The aluminum paste is transformed by firing from a dried state, 60, to an aluminum back electrode, 61. The backside silver or silver/aluminum paste, 70, is fired at the same time, becoming a silver or silver/aluminum back electrode, 71. During firing, the boundary between the back side aluminum and the back side silver or silver/aluminum assumes an alloy state, and is connected electrically as well. The aluminum electrode accounts for most areas of the back electrode, owing in part to the need to form a p+ layer, 40. Because soldering to an aluminum electrode is impossible, a silver back electrode is formed over portions of the back side as an electrode for interconnecting solar cells by means of copper ribbon or the like. In addition, the front electrode-forming silver paste, 500, sinters and penetrates through the silicon nitride film, 30, during firing, and is thereby able to electrically contact the n-type layer, 20. This type of process is generally called “fire through.” This fired through state is apparent in layer 501 of FIG. 1F.
JP-2001-313400A to Fujii et al. teaches a solar cell which is obtained by forming, on one main surface of a semiconductor substrate, regions that exhibit the other type of conductivity and forming an antireflection coating on this main surface of the semiconductor substrate. The resulting solar cell has an electrode material coated over the antireflection coating and fired. The electrode material includes, for example, lead, boron and silicon, and additionally contains, in a glass frit having a softening point of about 300 to 600° C., and one or more powders from among titanium, bismuth, cobalt, zinc, zirconium, iron, and chromium.
U.S. Pat. No. 4,737,197 to Nagahara et al. discloses a solar cell including a semiconductor substrate, a diffused layer provided in the semiconductor substrate by diffusion of dopant impurities, and a contact made of metal paste formed on the diffusion layer. The metal paste includes metal powder, which functions as the main contact material, glass frits, an organic binder, a solvent, and an element belonging to the fifth group of the periodic table.
U.S. Patent Publication 2006-0231801 A1 to Alan et al. teaches a thick film conductive composition comprising: (a) electrically conductive silver powder; (b) zinc-containing additive; (c) glass frit wherein said glass frit is lead-free; dispersed in (d) organic medium. As a preferred glass, the following composition is referred to: SiO2 0.1-8 wt %, Al2O3 0-4 wt %, B2O3 8-25 wt %, CaO 0-1 wt %, ZnO 0-42 wt %, Na2O 0-4 wt %, Li2O 0-3.5 wt %, Bi2O3 28-85 wt %, Ag2O 0-3 wt %, CeO2 0-4.5 wt %, SnO2 0-3.5 wt %, and BiF3 0-15 wt %.
U.S. Patent Publication 2006-0289055 to Sridharan et al. teaches a solar cell comprising a contact made from a mixture wherein, prior to firing, the mixture comprises: solids portion and organics portion, wherein the solids portion comprises (i) from about 85 to about 99 wt % of a conductive metal component, and (ii) from about 1 to about 15 wt % of a glass component, wherein the glass component is lead-free.
Although, as noted, various methods and compositions for forming solar cells exist, there is an on-going effort to provide compositions which are Pb-free while at the same time maintaining electrical performance. The present inventors create a novel composition and method of manufacturing a semiconductor device which provides such a Pb-free system and maintains electrical performance and solder adhesion.